This invention relates to a method for compressing picture data in order to reduce capacity of a memory and transmitting time for the data, in a picture reproducing machine.
In a conventional picture reproducing machine such as a layout color scanner, when the processing order of the data is changed, picture signals obtained by scanning an original picture photoelectrically are once stored in a memory using a magnetic disc, a magnetic tape, or the like, and then the picture signals read out of the memory are processed. In this case, therefore, a memory having a large capacity is required and a lot of processing time is also required.
For example, generally the scanning is carried out with the conditions of 200 scanning lines per centimeter in the direction of cylinder's axis and a sampling pitch of 5.times.10.sup.-3 cm in the direction of cylinder's periphery. Accordingly, when a picture image is reproduced on a photosensitive material of 70.times.50 cm, the entire picture elements required are calculated as follows. EQU 70.times.200.times.[50/(5.times.10.sup.-3)]=1.4.times.10.sup.8
If density data of a picture element is written by a binary code of eight bits (one byte), in order to store the entire picture elements required to such a photosensitive material, 140 megabytes of capacity is required for one color, and thus 560 megabytes for four colors. This method requires a memory having a large quantity of capacity and thus involves a long time for the processing.
In a conventional data compression method used in a facsimile, or the like, the picture element is compared with the following one along only one scanning line and is discriminated whether the two are the same or not one after another, viz., if one picture element is black, the following one is discriminated to be black or white, and then run lengths of element of the same color, i.e. black or white, are discriminated. However, this method is too simple to be applied to a picture having continuous tone in a color scanner, and thus this method is not practicable.